Yes, that can be done easily.
It's one of the basic features of LTE which allows you to shape your spectrum to a great degree. And unlike combining UMTS (WCDMA) and GSM you loose far less efficiency.
Huawei and Vodafone have run a trial in Spain that demonstrated GSM and LTE transmissions co-existing on the same spectrum. The idea behind Huawei's GL DSS (GSM-LTE dynamic spectrum sharing) is to let operators roll out their shiny new LTE infrastructure without restricting the spectrum available to the (currently) larger …
LTE allows you to vary the bandwidth with occupied by an uplink or downlink carrier, so I guess this works by having the operator define, say, a 20MHz LTE carrier but then vary it to 15, 10 or 5MHz during periods of peak GSM demand. The GSM cell would have a fixed BCCH carrier, configured on spectrum outside of the 20MHz used by LTE, and a number of traffic-only expansion carriers, which are assigned to carriers inside the LTE allocation.
when the LTE carrier slims down the GSM carriers can be switched on, as the LTE carrier beefs up again the GSM carriers are switched off again. Neighbouring GSM sites are allocated different channels within the 15MHz freed up by LTE to avoid co or adjacent channel interference.
I can see that working...but only if the operator had 20MHz to play with in the first place...